The present invention relates to vapor grown carbon fibers generally, and more particularly to vapor grown carbon fibers having high surface energy and high surface area, and to methods of producing such fibers without post-manufacture treatment.
Commercial carbon fibers hold great promise as a high performance material for composites due to their high strength and high modulus. They are commonly made by elevating a precursor material such as polyacrylonitrile (PAN) or pitch in an inert atmosphere to a temperature around 1000xc2x0 C. on continuous wind-up devices. They are generally continuous filaments and approximately 8 xcexcm diameter.
This application is concerned with vapor grown carbon fibers (VGCF), which are a relatively recent entry in the field of carbon fibers and have similar or even superior physical properties, along with the potential for production at a lower cost.
The concept underlying the production of VGCF begins with metallic particles, with iron generally being the predominant constituent, which catalyze the growth of long slender partially graphitic filaments when exposed to a hydrocarbon gas in the temperature range of 600xc2x0-1500xc2x0 C. It is known that sulfur enhances the growth (Kaufmann, U.S. Pat. No. 2,796,331) and that ammonia results in improved distribution in fiber diameter (Alig, et al. U.S. Pat. No. 5,374,415).
The iron catalyst required for the reaction is provided by using either liquid zero valent iron compounds such as iron pentacarbonyl or solid zero valent iron compounds dissolved in liquid hydrocarbons. An example of the latter is ferrocene. Liquid compounds are preferred since their introduction into the reactor is easier to control. They are most readily introduced by bubbling an inert gas through the liquid, thus carrying the catalyst into the reactor in the vapor state. The normal inert gases used for this method are argon, helium, and nitrogen.
U.S. Pat. No. 5,024,818 to Tibbetts et al. and U.S. Pat. No. 5,374,415 to Alig et al., the disclosures of which are hereby incorporated by reference, describe typical reaction processes and chambers. VGCF differ substantially from commercial carbon fiber in that VGCF are not continuous. They are about 0.001 to 0.04 mm long. Also, VGCF are much finer than their continuously grown counterparts. In addition, upon leaving the reactor, the fibers exist as an entangled mass that is very lightweight with a large apparent volume, from 5 to 50 ft3/lb. In other words, the fibers form a lightweight, fluffy entangled mass. In this state, the fibers are very difficult to ship and handle. Such a light and fluffy material is almost impossible to incorporate into mixing equipment that typically processes rubber or plastic. The fly loss and incorporation time are tremendous.
The VGCF produced by these processes have a surface energy in the range of 25 to 40 mJ/m2. This surface energy, although useful for some applications, is considered low with respect to the ability of resins to wet and, therefore, to adhere to the fiber for purposes of preparing useful composites. Due to the low surface tension, the fibers cannot be easily wetted out or mixed into liquid applications without prior surface treatments. These problems represent a severe limitation on the use of VGCF, as they cannot be readily dispersed into rubbers, plastics or the like. Thus, the development of methods by which the VGCF are wet out is important to the commercialization of these materials.
The conventional technique for providing the required surface modification of VGCF involves exposure to one or more of a number of oxidizing or etching agents in a post-manufacture processing step. xe2x80x9cSurface Properties of Carbon Fibersxe2x80x9d P. Ehrburger, in Carbon Fibers and Filaments, pp 147-161, J. L. Figueiredo, et al. (eds.) Kluwer Academic Publishers, 1990. xe2x80x9cEffect of Surface Treatment on the Bulk Chemistry and Structure of Vapor Grown Carbon Fibersxe2x80x9d, H.Darmstadt et al., CARBON, 35, no. 11, pp. 1581-1585, 1997. Such post-manufacture processing steps have also been demonstrated to provide desired modification of surface properties of VGCF. xe2x80x9cEffect of Surface Treatment on the Bulk Chemistry and Structure of Vapor Grown Carbon Fibersxe2x80x9d, H. Daimstadt et al., CARBON, 35, no. 11, pp. 1581-1585, 1997.
Carbon fibers may be activated by exposure to a number of oxidizing agents in the gas phase, including H2O and CO2. xe2x80x9cActivation of Carbon Fibers by Steam and Carbon Dioxidexe2x80x9d, S. K. Ryu, et al., CARBON 31, no. 5, pp. 841-842, 1993. Also, the use of ammonia as an additive gas during fiber synthesis, as taught by U.S. Pat. No. 5,374,415, has been shown to have beneficial impact on the morphology of the fiber, and to modify the surface of the fiber. Furthermore, the use of air containing O2 as a purge gas following the synthesis of the fiber has been shown to provide some degree of oxidation of the fiber.
However, the known processes for increasing the surface energy and surface area of VGCF require post-manufacture treatment, which increases the cost and complexity of production. Accordingly, there is a need for methods of making high surface energy VGCF without post-manufacture treatment. There is also a need to produce such fibers in a way that maintains the physical integrity of the fiber, i.e., the inherent fiber strength is not compromised, and other inherent fiber properties, such as electrical and thermal conductivity, are maintained.
These needs are met by the present invention whereby high surface energy VGCF and methods of making such fibers are provided. The high surface energy VGCF of the present invention have a surface energy greater than about 75 mJ/m2 without post-manufacture treatment. They preferably have a surface energy in the range of about 125 to about 185 mJ/m2 and more preferably about 145 to about 185 mJ/m2. In addition, they preferably have a total surface area in the range of about 25 to about 200 m2/g, and an external surface area greater than 20 m2/g. The surface area of the high surface energy vapor grown carbon fibers is preferably increased by a factor of at least 2, more preferably 10 or greater.
The invention teaches the use of a gaseous oxidant, such as CO2, as an additive during fiber synthesis in order to provide the desired surface modification to the fiber, including increased surface area and surface energy. Because CO2 is known to oxidize carbon as well as iron, the use of CO2 during fiber synthesis might be expected to poison the fiber synthesis reaction, or to degrade the mechanical properties of the fiber. Thus, the successful use of CO2 as a surface-modifying agent during a one-step fiber synthesis process without serious adverse effects on fiber synthesis rates or fiber structural properties was unexpected. This discovery reduces the cost and complexity of the production of high surface energy VGCF. It provides a one-step fiber synthesis process without the necessity of post-manufacture treatment.
In accordance with the present invention, methods of making a high surface energy VGCF are provided. One method involves forming a mixture comprising a gaseous hydrocarbon, ammonia, and an iron-containing compound decomposable to form iron nucleation sites. The hydrocarbon and the ammonia are present in an amount sufficient to provide a ratio of carbon atoms to nitrogen atoms in a range of from about 1:1 to 30:1. The gaseous oxidant is added as a separate stream to the mixture. The mixture is heated in a reactor for a time and at a temperature sufficient to cause decomposition of the decomposable compound to form particles of nanometer size iron nucleation sites dispersed and entrained in the gaseous mixture which induce growth of carbon fibers. High surface energy VGCF are formed, which have an average diameter of about 0.05 to about 0.5 micron, contain carbon and nitrogen, and have a surface energy greater than about 75 mJ/m2. The high surface energy VGCF are then recovered.
In another embodiment, the carbon dioxide is introduced into the mixture by using it as a carrier for the iron-containing compound.
Accordingly, it is an object of the present invention to provide methods of making high surface energy VGCF without post-manufacture treatment. Another object of the invention is to provide high surface energy VGCF having a surface energy greater than about 75 mJ/m2 without post-manufacture treatment. These and other objects and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appealed claims.